The invention relates generally to a gas detector and more particularly to a gas detector for power plant emission measurements and control.
Gas detectors have been used in various industrial applications such as power plants, transportation vehicles, gas turbines, and industrial, environmental, and biological processes and installations. A gas detector typically detects the presence of one or more gases in a given area. In some applications, detectors can further measure concentrations of specific gases.
In recent years, utility deregulation and increasing electrical demand have led to the integration of solar and wind power with combined cycle (CC) power plants. This integration has led to a desire to reduce start-up and shutdown times for the gas turbines in the CC power plants. The emission levels from a gas turbine vary depending on the manner in which the machine starts up or shuts down. Often, emissions from power plants may deteriorate as a result of seasonal changes, changes in fuel quality, and deterioration of gas turbine airfoils. A continuous emission measurement system (CEMS) comprising a gas detector measures gaseous emission species such as NOX (for example Nitrogen monoxide NO and Nitrogen dioxide NO2), carbon monoxide (CO) and ammonia (NH3) for continually demonstrating compliance to permitted emission limits.
A CEMS coupled with a gas detector typically uses an extractive process of sampling out near homogeneous portions of the exhaust gas from the exhaust gas duct, a heat recovery steam generator (HRSG) coupled to the exhaust gas duct, and a gas exhaust stack. This process typically requires from about 30 seconds to about 3 minutes of measurements as well as frequent purging and calibration processing. A CEMS typically uses measurement techniques such as interaction of light with the emission species like light absorption, chemi-luminescence, and/or non-dispersive infrared analysis. In light absorption based techniques, light beams of particular wavelengths, at which the respective emission species to be detected have peak absorption, are transmitted through an area where the emission species are expected to be present. One or more photo detectors then receive the transmitted light beams. Various types of photo detectors are available commercially and are chosen based on the wavelength of the light beams employed in detecting the gases. By employing Bear-Lambert's law and other correction and conversion factors, a controller can convert the photo detector output signal to provide concentration information of the specific gas or gases of interest and may further control and manage emissions.
Power generation plants require reliable, accurate, and timely detection and measurement of multiple gases. However, the harsh and challenging operating conditions of power generation plants may adversely affect selectivity and sensitivity in light absorption spectroscopy measurements. For example, during transient operations rapid changes of NOx levels can occur, and the CEMS technology will introduce a time delay in the NOx measurement. In such situations, ammonia injection may be incorrectly high or low depending on the actual NOx change during the time delay. A high ammonia injection in selective catalytic reduction (SCR) system leads to the formation of corrosive ammonium sulfates adversely impacting downstream hardware. A low ammonia injection in SCR system may lead to un-necessarily high stack NOx because not enough ammonia was injected when needed. In addition, many challenges exist for in-situ monitoring of power plant exhaust emission species such as the presence of moisture, harsh environment, high temperatures, and dynamic fluctuations in trace gas concentrations.
Accordingly, there is a need for simultaneous, in-situ, real time, selective, and sensitive measurement of multiple emission species to reduce and control startup and transient emissions in power plants. Further it would be desirable to provide reliable measurements with greater accuracy, repeatability, and consistency to demonstrate compliance or attainment to regulatory emission levels.